This invention relates in general to two-way radios and more particularly to the protection of two-way radio circuitry from high radio frequency (RF) input levels.
Two-way radio equipment is commonly used for many types of everyday functions such a police, fire, government and military applications. This equipment can range from small portable radios to the more complex higher power base station equipment. The higher power equipment is essentially that equipment used in either mobile or base station applications.
A problem often occurs when two or more high-power radios are in close proximity i.e. less than 10 feet apart. This can present itself when two or more mobile radios are positioned very close to one another or two high-power base station radios are used with antennas that are positioned in close proximity. When this occurs, the radiated radio frequency (RF) energy from the nearby transmitting radio can overload the receiver front-end circuitry of any radio in idle or stand-by mode. This can cause either immediate or long-term degradation in the sensitivity of the nearby receiver.
The impact of the overload condition will depend on any number of factors. These include the level of the transmitter power, frequency of operation of either receive or transmit radio, proximity of the antenna system and the duration of the overdrive conditions. Generally speaking, the higher the transmitted power from the offending radio, the more severe the degradation in the second receiver. The frequency of operation of either receive or transmit radio also plays a role in the potential damage to the receive radio. For instance, if receive and transmit radios belong to different bandsplits, (i.e. VHF vs. UHF), the potential for damage is reduced since the preselector in front of the receive amplifier device will knock down the level of the interfering signal that reaches the pre-amplifier device. In fact, the worst-case scenario occurs when the two or more high-power units operating in close vicinity belong to the same bandsplit (i.e. either VHF, UHF or 800MHz). In this particular instance, the overloading signal falls within the passband of the preselector in front of the pre-amplifier device, thereby exposing the pre-amplifier device in the receive radio to the full blast of the interfering signal. Another factor in this issue has to do with how long the receiver is exposed to the overdrive conditions. Eventually, the longer the transmitter remains keyed up, the greater the potential for damage to the pre-amplifier device. The net effect of the front-end overload is to reverse-bias the base-emitter junction of the amplifier device and, as a result, the direct current (DC) gain of the device will degrade. As the DC gain of the device degrades, so does the receive radio sensitivity. Ultimately, the amplifier device would have to be replaced causing unneeded repair time and expense.
As seen in prior art FIG. 1, this problem has been dealt with in the past by implementing a system 100 using some type of push-to-talk (PTT) relay or T-R switch. A first high power radio transceiver 101 is used in connection with a relay 103a. When the first radio transceiver 101 is in transmit mode, the relay is keyed or positioned such that RF energy is output to its associated antenna 105. With another radio transceiver 109 in close proximity, the Push-To-Talk (PTT) signal from transceiver 101 is used to position the relay 103b such that antenna 111 is no longer directly connected to the radio transceiver 109. This prevents powerful stray RF energy from entering the receiver front end of radio transceiver 109 and causing damage to the front-end device.
Either relay 103a or 103b is toggled through the PTT function of either radio transceiver assuming an operator will be transmitting on only one radio transceiver at a time. As seen in FIG. 1, when each respective radio transceiver is not connected to its antenna, the RF energy captured by the antenna is directed away from the radio from the radio front end towards a non-reactive load (107, 113).
The problem with this type of system is that it requires attachment of relays 103a, 103b and prevents one of the radio transceivers from operating properly when its antenna is not connected due to operation of the other transceiver. Another limitation of the system described above is that it relies exclusively on the transmit status of the nearby transceiver to activate the relay. If a third transceiver were to enter the picture and transmit nearby, both transceivers 101, 109 would be left unprotected because the transmit status of that third unit would be unknown to relays 103a, 103b. 
Thus, the need exists to provide a method of receiver overload protection that is integrated into the radio transceiver in order to prevent damage and/or long term degradation in receiver sensitivity without the need for an external relay or switch to disconnect the antenna of another transceiver in close proximity.